When they come to write the authorised version of this lush age of children's writing, there will be pages and pages devoted to celebrities who suddenly realised that all along, their true vocation had been to tell stories to kids. It is under that chapter heading that the following tale will be told: the story of how the Saint was saved.

The Saint, aka Simon Templar - as those of you who stayed off school in the 70s to catch what passed for daytime TV back then will remember - was no slouch himself when it came to the debonair rescuing of folk in the nick of time. A Robin Hood hero, he broke the law in the name of greater justice. Where he strayed, we followed.

Yet so fickle are we television viewers that those we have loved often end up languishing. The Saint lives on only in web-land, and you have to be a fan to find the site. Which means there hasn't been much work, either, for the actor who played him: one Ian Ogilvy.

Except that, at the age of 60, zap! - Ogilvy is suddenly sought after again. Deftly - of course - he has switched careers and is now the author of a fast-selling children's story, Measle and the Wrathmonk. The fact that Roger (Back to the Future) Zemeckis has optioned it is pure icing.

Measle and the Wrathmonk is about a young boy sent to live with his dastardly uncle. Said relative, it turns out, is a warlock gone bad: a wrathmonk, a word and conceit dreamed up by Ogilvy. This corrupted evil genius plays with humans, turning the ones he doesn't like into tiny plastic models for toying with in his train set.

It is late evening when I meet Ian Ogilvy in the purple, throbbing basement of a bar in central London. The features are the same, if podgier, the hair streaked now, but the suit, the shirt, the shoes are of course immaculate.

One barely has to squint to see the pencil drawing of the halo above the head, the arm crooked at the elbow, hand jauntily on hip. It is really him: the one who took over from Roger Moore, the returned Saint.

"I'm the luckiest puppy," he says. "I got a jolly good deal - I'm a bit coy about figures, but it went to auction. Now I'm in the process of selling it all over the place: France, Brazil, Israel, Germany. I had a guy representing it, a New Yorker, who sells books to movies. Bang! he called me, said, we've got a deal, and a good one [he's telling me this in a kind of playground singsong voice now: la, la, lalala]. Robert Zemeckis and Warner brothers - it doesn't get better than that."

Zemeckis bought the rights to the book because the story suits the newest form of animation technique. This is where actors don't just provide voices for cartoons, but actually perform the character. The actor wears a costume with reflective dots on it, which the camera then picks up to make computer-generated images - the same method that was used to create Gollum in The Lord of the Rings.

The film of Measle and the Wrathmonk comes out in 2005, and plans for a sequel are already under way. Ogilvy has written the next Measle story and is working on the third.

The first book was written over three years, stuffed in and pulled out of drawers. Now Ogilvy is on contract for the next two, and it's tougher. But he is forcing himself to write a thousand words at a time, and is strict about doing two thousand a week: "You can make a book like that." He says he is particularly enjoying making up the names of the seven wrathmonks who wreak revenge on tiny Measle in book two.

It was reality TV, Ogilvy thinks, that did for him and his ilk. "I've got friends in the profession who are selling their houses," he says. "None of us saw it coming; none of us realised that the general public preferred to see the general public, and producers certainly prefer to use the public - you don't have to pay them, and you've got ratings going through the roof. I made a decent living, and overnight it stopped.

"Then it suddenly becomes cute for movie stars to do guest spots on TV shows. Where before a movie star would not be seen dead on television, all of a sudden you've got Brad Pitt on Friends, Glenn Close on ER, and those were the parts that actors like me got, once. Now they don't make the shows, and if they do, they can get anybody they want.

"I can't blame them, but it's not very nice. Frankly, the writing tail is wagging the acting dog now, for me."

So, uh, Brad, honey, don't hold your breath for that call from Zemeckis. Not if the author has any say over the casting, anyhow. Mary Tyler Moore? Now there's a thought.